AAIW is a low salinity water mass that fills most of the southern hemisphere and the tropical oceans at about 800 to 1000 m depth. As the densest of the circumpolar Subantarctic Mode Waters (SAMW), the salinity minimum that lies at the top of the AAIW layer is formed as a thick, outcropping mixed layer in the southeastern Pacific just north of the Subantarctic Front (SAF). SAMW and AAIW formation have a major impact on the oceanic sink for anthropogenic CO2, whose largest uncertainty is at intermediate depths. In 2005 and 2006, we received NSF funding to carry out an austral winter and subsequent austral summer cruise to characterize the deep mixed layers in the SAMW/AAIW formation region in the SE Pacific.
Shipboard measurements included CTD, salinity, oxygen, nutrients, XCTD, ADCP, LADCP, shipboard meteorology, CFCs, CO2, underway N2O and CO2, underway T/S/O2. We deployed floats for ARGO and a surface drifter with a pCO2 sensor.
Subsequent to the cruises, in addition to analysis of these data sets, we used this funding and subsequent NSF funding (I. Cerovecki, M. Mazloff, L. Talley) to analyze output from the Southern Ocean State Estimate to provide a global context for formation of the SE Pacific SAMW.
Full text of NSF project summary

Images for choosing special survey regions
As of August 19, these suggest that the SAF has been shifted northward throughout
the winter in the target region for the convection survey. Therefore we might
expect deepest mixed layers to also be shifted northward (north of the SAF).
SST pattern indicates a developing ridge of cold water, just north of the
current location of the SAF (based on altimetry). Hence, as of today (prior
to any CTD profiling for verification), it would appear that the convection
study should be located at the breakpoint station at 77W (centered at 57S, 77W).
August 20 update: microwave SST map shows weaker low temperature (not
apparent in coarse colloring in Goni maps). However, lowest temperature is
still at the first leg breakpoint.
August 21 and 22 update: MW SST shows warming of surface.
Aug. 24: deep mixed layers detected (8/24/05)
Aug. 26: Subantarctic Front detected and second intensive survey planned (8/26/05)

Carter, B.R., L. D. Talley and A. G. Dickson, 2013. Mixing and remineralization in waters detrained from the surface into Subantarctic Mode Water and Antarctic Intermediate Water in the southeastern Pacific. Submitted to J. Geophys. Res. Oceans.

Firing, Y.L., 2012. Structure and dynamical balance of the Antarctic Circumpolar Current in Drake Passage. PhD thesis. University of California, San Diego. 171 pp. (Partial support from this grant) (T. Chereskin, advisor)

Lenn, Y.-D., 2006. Observations of Antarctic Circumpolar Current dynamics in the Drake Passage and small-scale variability near the Antarctic Peninsula. (Partial support from this grant) (T. Chereskin, advisor)

Subduction into the southern hemisphere subtropical gyres is
dominated volumetrically by Subantarctic Mode Water and the
densest of the SAMW's, Antarctic Intermediate Water (AAIW).
SAMW refers to the thick mixed layers found directly
north of the Subantarctic Front. A map of a proxy of winter
time mixed layer depth (
Fig. 1 from Talley, 1999) shows that these layers are
thickest in the eastern Indian Ocean and all across the Pacific.
The sudden onset of thicker layers in the central Indian Ocean
is a mystery to be solved.

In the Pacific Ocean, the densest subducting layer is
the densest of the global SAMWs, and is identical there to
AAIW. AAIW is formed in the
southeastern Pacific, just west of southern Chile, from where it
spreads to the entire southern hemisphere and tropics throughout
the globe (schematic in
Fig. 3, from Talley, 1999).
The Pacific AAIW salinity minimum originates from this single
source region, through northward subduction. The AAIW found in
the Atlantic and Indian originates primarily from this source as
well, as a portion of the new AAIW flows eastward through Drake
Passage, north of the Subantarctic Front. Some modification of the
AAIW takes place during this passage and then it spreads into the
South Atlantic's subtropical gyre through eddy shedding and mixing
out of the Falkland (Malvinas) loop. (This is not a wind-driven
subduction process.) Based on oxygen, potential vorticity and
salinity in the Atlantic and Indian Oceans, there is no source of
AAIW in these oceans other than the southwestern Atlantic.
Hence the primary site to study for AAIW formation is the
southeastern Pacific, and the primary sites to study for AAIW
spreading are Drake Passage and a subtropical section such as the
quarterly XBT line at 30S in the Pacific.

Why are these dense and volumetrically large subducting water
masses important for climate? They form a large fraction of the
subducted water of the subtropical gyres. They spread northward and
cross the subtropical/tropical potential vorticity barrier and are
found in a weaker form in the tropics. Variations in Southern Ocean
surface properties are therefore communicated throughout the
subtropical southern hemisphere and into the tropics through these
water masses. In the Atlantic, AAIW is a marked part of the warm
layer that moves northward, feeding the North Atlantic
thermohaline overturning.

For several decades there have been two separate ideas about SAMW and
AAIW formation.

(a) The older concept was that AAIW is formed
circumpolarly, through Ekman transport northward across the Subantarctic
Front and then sliding northward down isopycnal surfaces.

(b) A more recent concept (still dating to the 1970s, McCartney, 1976) is
that SAMW is formed north of the Subantarctic Front through subtropical
gyre processes and that AAIW is the densest of the SAMWs, and hence
localized in its formation area. (SEISAMW has been recently
recognized as a similarly important water mass with respect to its
ventilation of the Indian Ocean. )

Both mechanisms are likely active. The northward Ekman transport
across the SAF occurs into the local SAMW layer and may be important for
creating the great thickness of the SAMW layer. WOCE hydrographic
sections show intrusions across the SAF into the SAMW. Eastward
transport of the SAMWs then eventually leads to both new SEISAMW,
subducting into the Indian Ocean, and new AAIW, partially subducting
into the Pacific Ocean and partially passing on to the Atlantic/Indian
subtropical gyre. The second source of water for the SAMW and AAIW is
subtropical circulation poleward in the western boundary currents, in
the traditional model of subtropical subduction.

In the region south of Australia, Rintoul et al (JPO, submitted) have
looked at correspondences between Ekman variability, SAMW properties and
atmospheric variability and found that Ekman transport across the SAF should
not be ignored.